Chapter 104: Hydrofluoric Acid and Fluorides

HISTORY AND EPIDEMIOLOGY

Hydrofluoric acid has been known for centuries for its ability to dissolve silica. The Nuremberg artist Schwanhard is given credit for the first attempt in 1670 to use HF vapors to etch glass.⁴⁷ Today, hydrofluoric acid (HF) is widely used throughout industry. In addition to glass etching, HF is used in brick cleaning, etching microchips in the semiconductor industry, electroplating, leather tanning, rust removal, and the cleaning of porcelain.⁴⁷ From 2011 to 2015, the American Association of Poison Control Centers (AAPCC) reported 2,761 single-substance exposures to HF and 6 deaths (Chap. 130). The hands are the commonest part of the body injured. Exposures to HF often occur as unintentional occupational hazards. The actual number of work-related poisonings from HF appears difficult to quantitate because of limitations in International Classification of Diseases (ICD) medical coding and the lack of notification of regional poison control centers by worksites.¹⁰

Hydrofluoric acid is also the commonest cause of fluoride poisoning, although other forms of fluoride, including sodium fluoride (NaF), ammonium bifluoride (NH₄HF₂), and sodium or zinc fluorosilicate, also produce significant toxicity. Historically, NaF was used as an insecticide, rodenticide, an antihelminthic for swine, and a delousing powder for poultry and cattle. Ammonium bifluoride is mainly used in industrial inorganic chemistry, especially in the processing of alloys and in glass etching. Other fluoride salts are widely used in, for example, the steel industry, drinking water, toothpaste additives, electroplating, lumber treatment, and the glass and enamel industries.

The widespread use of HF and fluoride-containing compounds results in significant toxicity. In 1988, an oil refinery in Texas released a cloud of hydrogen fluoride gas that resulted in 939 people seeking hospital treatment and 94 of these patients requiring admission.¹⁰⁰ The petroleum industry has since been plagued by similar HF incidents.¹⁰⁰ In 2015, Washington State reported one death and 48 occupational HF burns associated with car and truck washing between 2001 and 2013.⁷⁸ Sodium fluoride was responsible for the poisoning of 263 people and 47 fatalities when it was mistaken for powdered milk and unintentionally combined with scrambled eggs.⁵⁵ Following ingestion, this and other fluoride salts are converted to HF in vivo, resulting in significant fluoride toxicity.

CHEMISTRY

Hydrofluoric acid is synthesized as the product of gaseous sulfuric acid and calcium fluoride, which is subsequently cooled to a liquid.⁵⁶ Aqueous HF is a weak acid, with a pK_a of 3.17; as such, it is approximately 1,000 times less dissociated than equimolar hydrochloric acid, a strong acid. Hydrofluoric acid generally ranges in concentrations from 3% to 40%, for use in both industry and the home. Anhydrous HF is highly concentrated (>70%) and used almost exclusively for industrial purposes. Hydrofluoric acid has unique properties that can cause life-threatening complications following seemingly trivial exposure.

Sodium fluoride is commonly ...